vaapi_h264: Add workaround for bad SEI in old Intel drivers

With pre-2.0 Intel drivers in CBR mode, if an explicit SEI message with
the old (now deprecated) type is not included, the driver generates and
inserts some timing SEI which is almost certainly invlaid.  Before
7a4fac5e91 we always inserted our own SEI
so this would not be visible, but since then it has been possible to
disable that.  We would also like to avoid using the deprecated type,
and using the new type, while working in old drivers, does not suppress
the spurious message like the old type does.

Therefore, suppress the bad SEI insertion by providing a zero-length
buffer with the old type, which the driver can insert harmlessly.

libavcodec/vaapi_encode_h264.c

@@ -82,6 +82,7 @@ typedef struct VAAPIEncodeH264Context {
     CodedBitstreamFragment current_access_unit;
     int aud_needed;
     int sei_needed;
+    int sei_cbr_workaround_needed;
 } VAAPIEncodeH264Context;
 
 typedef struct VAAPIEncodeH264Options {
@@ -254,6 +255,19 @@ static int vaapi_encode_h264_write_extra_header(AVCodecContext *avctx,
 
         *type = VAEncPackedHeaderH264_SEI;
         return 0;
+
+#if !HAVE_VAAPI_1
+    } else if (priv->sei_cbr_workaround_needed) {
+        // Insert a zero-length header using the old SEI type.  This is
+        // required to avoid triggering broken behaviour on Intel platforms
+        // in CBR mode where an invalid SEI message is generated by the
+        // driver and inserted into the stream.
+        *data_len = 0;
+        *type = VAEncPackedHeaderH264_SEI;
+        priv->sei_cbr_workaround_needed = 0;
+        return 0;
+#endif
+
     } else {
         return AVERROR_EOF;
     }
@@ -608,6 +622,10 @@ static int vaapi_encode_h264_init_picture_params(AVCodecContext *avctx,
 
     if (opt->sei & SEI_IDENTIFIER && pic->encode_order == 0)
         priv->sei_needed = 1;
+#if !HAVE_VAAPI_1
+    if (ctx->va_rc_mode == VA_RC_CBR)
+        priv->sei_cbr_workaround_needed = 1;
+#endif
 
     if (opt->sei & SEI_TIMING) {
         memset(&priv->pic_timing, 0, sizeof(priv->pic_timing));